Perpendicular magnetic recording (PMR), in which the recorded bits are stored in the generally planar recording layer in a generally perpendicular or out-of-plane orientation, is a promising path toward ultra-high recording densities in magnetic recording hard disk drives.
A PMR write head (writer) and a PMR read head (reader) are typically formed as an integrated read/write head on an air-bearing slider. The slider is attached to an actuator arm by a suspension and positioned very close to the disk surface by the suspension. The actuator moves the slider across the disk surface so that the read/write head can access the data tracks. There are typically a stack of disks in the disk drive with a slider-suspension assembly associated with each disk surface in the stack.
In a PMR writer, a write current passes through a writer coil disposed adjacent to a magnetic yoke to induce a strong write magnetic field at a write pole to write data in a recording media. The recording layer has perpendicularly recorded magnetizations or magnetized regions that form a data track, with adjacent regions in the data track having opposite magnetization directions. As long as the write magnetic field at the write pole is strong enough, data written in the recording layer can be erased or “overwritten”. In fact, the overwrite operation is a popular way to erase data in a magnetic recording hard drive. However, if the induced magnetic field is not strong enough, the overwrite or erasing may not be fully effectuated in all magnetic regions, thereby causing data errors in subsequent recordings.
In a PMR reader, a tunnel magnetoresistance (TMR) sensor is frequently employed in the read head. The TMR sensor includes a patterned TMR structure or stack having two ferromagnetic layers separated by an insulating (e.g., MgO) barrier layer. One ferromagnetic layer is magnetically oriented in a fixed direction (the “pinned layer”) and the other ferromagnetic layer rotates in response to an external magnetic field (the “free layer”). The TMR sensor also includes a hard bias layer disposed on either side of the TMR stack. The hard bias layer includes a permanent magnetic material, such as cobalt platinum (CoPt), and provides a bias field along a direction perpendicular to layers of the TMR stack. The resistance of the device is dependent on the relative orientation between the two ferromagnetic layers. In a perpendicular magnetic recording (PMR) read head, a sense current passes perpendicularly through layers of the TMR stack. The magnetic transitions between adjacent oppositely-directed magnetized regions cause changes in electrical resistance that are detected by the TMR sensor.
The amplitude of a readback signal of a PMR reader can be asymmetric. Readback signal amplitude asymmetry means that the amplitude of the pulses from magnetizations recorded in one direction (e.g., the “positive” direction) is different from the amplitude of the pulses from magnetizations recorded in the opposite direction (e.g., the “negative” direction). The amplitude asymmetry (AASY) measured in percent can be expressed as [(SP−SN)/(SP+SN)]*100, where SP represents the measured amplitude of the pulses from magnetizations recorded in one direction and SN represents the measured amplitude of the pulses from magnetizations recorded in the other direction. A high value of AASY is undesirable in that it has a deleterious effect on the stability of the read head by causing a high bit error rate (BER) when the data is read back.
To some degree, AASY is a result of the construction of the reader It is, however, also believed that spurious magnetic fields arising from the media background and other sources also contribute to amplitude asymmetry. For example, rapidly increasing track density requires decreasing reader track width. As the reader track width decreases, de-magnetic fields from P1 and P2 layers rapidly increases, and the increased de-magnetic field, in turn, causes reader stability degradation by increasing the AASY.
A conventional scheme for improving the reader stability is to increase the hard bias field by increasing the thickness of the hard bias layer. However, with increasing track density, there is a requirement to reduce a shield-to-shield spacing for the TMR sensor stack. Therefore, it is often neither desirable nor practical to increase the hard bias layer thickness to improve the ASSY. In addition, an increase in the hard bias thickness causes a significant reduction in the reader amplitude which may not be a good compromise for the improved reader stability.
Therefore, a need exists for a scheme for improving the reader stability without a structural change to the existing read head design (e.g., an increase in hard bias layer thickness). In addition, a need also exists to improve the overwrite performance of a writer by increasing the write magnetic field at the write pole without a structural change to the existing write head design.